What processes can cause the surface of the Earth to rise? There are many because, as we have seen, mountain building happens in numerous different geologic settings. The lithosphere, which consists of relatively rigid crust and lithospheric mantle, “floats” on the softer asthenospheric mantle below. As a consequence, the elevation of the top surface of the lithosphere, over a broad region, represents a balance between buoyancy force pushing lithosphere up, and gravitational force pulling the lithosphere down. Geologists refer to the condition that exists when this balance has been achieved as isostasy, or isostatic equilibrium. Put another way, isostasy exists where the elevation of the Earth’s surface reflects the level at which the lithosphere naturally floats. (Note that because asthenosphere flows only very slowly, and because lithosphere is strong enough to hold up loads, isostasy does not exist everywhere).
To picture the relation between isostasy and mountains, imagine placing a block of wood into a bathtub full of water. If the block is less dense than water, it floats, with part of the block remaining above the water surface, and most of the block submerged below. Now, place a denser block of the same thickness next to the first block. The top of the denser block sits lower than that of the less-dense block of the same thickness. Similarly, the top surface of a thicker block sits higher than the top surface of a thinner block of the same density. If you were to add another block of wood on top of one that is already floating, the lower block would sink to adjust for the addition, so as to maintain isostatic equilibrium.
From our bathtub experiment, we can deduce that any phenomenon that changes the thickness and/or density of a floating block will affect the elevation of the block’s surface above the water surface. Since the lithosphere floats on the asthenosphere, the elevation of the lithosphere’s surface depends on the thickness and density of the lithosphere. So to answer the question of why mountain belts can rise, we must identify geologic processes that can change the thickness and/or density of layers in the lithosphere. Let’s consider some examples of how these charges take place.
Crustal shortening and thickening
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| The concept of isostacy as applied to the collisional mountain ranges. |
During collisional orogeny or during certain types of convergent-margin orogeny, horizontal compression causes the crust to shorten horizontally and thicken vertically. In fact, the folding, faulting, and plastic flow that take place during such events can almost double the crust’s thickness. For example, the crust beneath the tallest range, the Himalayas, is 70 km thick, whereas crust beneath the plains of the central United States is 35 km thick (figure above a). To isostatically compensate for the thickening of the crust (the geologic equivalent to adding another block of low-density wood to the top of a floating block), the base of the crust and underlying lithospheric mantle subside (figure above b). Indeed, since the Himalayas are about 8 km high, most of the thickened crust extends downward beneath the range just as most of an ice cube lies under water. This downward protrusion of crust is called a crustal root. We can illustrate this relationship in a bathtub model by lining up a row of floating blocks of different thickness if all the blocks have the same density, the thicker blocks rise farther above water and protrude deeper below the surface (figure above c).
Adding igneous rock to the crust
When lava and/or pyroclastic debris is deposited onto the surface, a volcano grows and may become a mountain. Growth of mountains associated with igneous activity may also occur because intrusions at depth may add material to the crust and, therefore, thicken it.
Removal of lithospheric mantle
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| Uplift, due to delamination of the lithosphere root, may happen after collision. |
The weight of the lithospheric mantle (composed of very dense rock) pulls the lithosphere down, just as heavy ballast makes a ship settle deeper into the water. Removal of some or all of the lithospheric mantle from the base of a plate, therefore, causes the surface of the remaining lithosphere to rise to maintain isostasy, even if the thickness of the crustal component remains unchanged (figure above a, b). Such removal, a process known as delamination, resembles removal of ballast from the hold of a ship as the weight of the ballast disappears, the deck of the ship rises.
Thinning and heating the lithosphere
In rifts, the lithosphere undergoes stretching and thinning. As a result, relatively less-dense asthenosphere rises beneath the rift, and the remaining lithosphere heats up. Replacing dense lithospheric mantle with less-dense hot asthenosphere, and heating the remaining, overlying lithosphere (thereby causing rocks to expand so their density increases) results in uplift of the rift and its borders.
What Goes Up Must Come Down
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| Manifestations of erosion in mountain ranges. When land rises, water and ice start cutting into it. |
When the land surface rises significantly, for whatever reason, it doesn't remain a smooth welt or bulge on the Earth’s surface. As soon as a difference in elevation between one location and an adjacent one develops, gravity begins to drive a variety of erosive processes. For example, as slopes steepen, landslides of various types cause rock and debris to tumble from higher to lower elevations; when rain falls, streams form and sculpt valleys and canyons; and if it remains cold enough, glaciers grow and flow, carving peaks and deepening valleys. The net result of all these processes is to grind away elevated areas and produce the jagged landscapes that we associate with mountain terrains (figure above a, b). It’s important to keep in mind that uplift and erosion happen simultaneously in active mountain belts, so for the elevation of a range to increase over time, the rate of uplift must exceed the rate of erosion.
The highest point on Earth, the peak of Mt. Everest, lies 8.85 km above sea level can our planet’s mountain ranges get significantly higher? Probably not. Mountains as high as Olympus Mons on Mars, which rises 27 km above the plain at its base, couldn't form on Earth because of the relatively high geothermal gradient (the rate of increase in temperature with depth) in Earth’s crust. Due to the gradient, quartz-rich crustal rocks at mid-crustal depths (15–30 km) become so warm and weak that they can flow ductilely. When this flow begins, overlying mountains above begin to collapse under their own weight, and spread laterally like soft cheese that has been left out in the summer sun. Geologists call this process orogenic collapse. During orogenic collapse, the upper crust breaks and a system of normal faults develop, to accommodate the horizontal stretching.
